准噶尔盆地彩南地区天然气干燥系数异常高原因分析

为明确准噶尔盆地彩南地区天然气干燥系数普遍偏高的原因，厘清天然气运移成藏规律，开展了天然气组分和碳同位素、储层中岩石矿物组成、方解石全岩碳氧同位素分析以及实验室烃类氧化模拟实验。该区侏罗系天然气以甲烷为主，干燥系数普遍大于0.95，δ¹³C₁值基本大于-32‰，C₇轻烃中以甲基环己烷占优势，甲基环己烷指数大于50%，指示来源于高—过成熟石炭系烃源岩。从天然气运移判识指标ln（C₁/C₂）与δ¹³C₁-δ¹³C₂图版来看，从彩47井区—彩31井区—彩003井，ln（C₁/C₂）值逐渐增大，但δ¹³C₁-δ¹³C₂值并未呈现变小或变大趋势，说明运移或成熟度并不是研究区天然气组分与碳同位素变化的主控因素。烃类热氧化模拟实验显示，油气中醇类在125℃时会被MnO₂氧化生成甲烷和二氧化碳，当温度达到200℃时才能氧化甲烷生成CO₂，从而改变油气的组成，造成天然气中甲烷含量增高。运用背散射电子探针技术，发现研究区侏罗系干气层段具有两类方解石，其中一类方解石中Mn含量较高，Mn含量可高达3%，在阴极发光下呈现亮橘红色、橘黄色特征，且方解石全岩碳同位素负偏，较正常方解石全岩碳同位素负偏5‰~10‰，随着方解石Mn含量越高，其全岩碳同位素越负，证实该区侏罗系干气层段存在普遍的弱烃类氧化作用。彩南地区侏罗系天然气干燥系数异常高的原因是腐植型烃源岩经过高成熟—过成熟演化后，在富含氧化性矿物的侏罗系储层中聚集成藏，后遭受烃类氧化作用，甲烷含量进一步增加，造成天然气干燥系数普遍偏高。

Reason for abnormally high drying coefficient of natural gas in Cainan area,Junggar Basin

In order to clarify the reason for generally high drying coefficient of natural gas in Cainan area of the Junggar Basin and find out the law of natural gas migration and accumulation, the analysis of natural gas components and carbon isotopes, rock mineral composition in reservoir, bulk carbon and oxygen isotope of calcite, and laboratory hydrocarbon oxidation simulation experiments were carried out. The Jurassic natural gas in Cainan area is dominated by methane, with drying coefficient of generally greater than 0.95, and δ13C1 value of basically greater than -32‰. Among C7 light hydrocarbons, methylcyclohexane is dominant, with methylcyclohexane index of greater than 50%, indicating that natural gas in the study area came from high-over mature Carboniferous source rocks. Judging from natural gas migration identification index of ln(C1/C2) with δ13C1-δ13C2, from well block Cai-47 to well block Cai-31, and then to well block Cai-003, ln(C1/C2) values gradually increased, but δ13C1-δ13C2 values did not show a trend of decreasing or increasing, indicating that migration or maturity is not the main controlling factor for the changes of natural gas composition and carbon isotope in the study area. Hydrocarbon thermal oxidation simulation experiments showed that alcohols in oil and gas were oxidized by MnO2 to generate methane and carbon dioxide at 125℃, and methane could only be oxidized to generate CO2 when the temperature reached 200℃, thus changing the composition of oil and gas and increasing methane content in natural gas. Using backscattered electron probe technology, it was found that there are two types of calcite in Jurassic dry gas interval in the study area. One type of calcite has a high Mn content, which can be as high as 3%. It appears bright orange and orange under cathodoluminescence. In addition, the bulk carbon isotope of calcite is negatively biased, which is 5‰-10‰ more negative than that of normal calcite. The higher content of Mn in calcite, the more negative of bulk carbon isotope, confirming that there is widespread weak oxidation of hydrocarbons in the Jurassic dry gas interval in Cainan area. Comprehensive analysis suggested that the reason for abnormally high drying coefficient of Jurassic natural gas in Cainan area is that the humic source rocks have undergone high-over mature evolution to generate natural gas that accumulated in Jurassic reservoirs rich in oxidizing minerals. Then the oxidation of hydrocarbons caused methane content in natural gas to increase further, resulting in a generally high drying coefficient of natural gas.